1. Field of the Invention
The present invention relates to a method for the production of carbon coated nanoparticles and carbon nanotubes by catalytic disproportionation of carbon monoxide and the nanoparticles and highly oriented carbon nanotubes produced from the method.
2. Discussion of the Background
Catalyzed deposition of carbon from the gas phase results in a number of different morphologies. The form in which carbon deposits can be of considerable practical importance (Rodriguez, J. Mater. Res. 8, 3233 (1993)).
Carbon deposition can foul reactors or deactivate catalysts. On the other hand, materials with special properties such as high surface area, conductivity, or high strength or modulus can be produced under appropriate conditions. The factors controlling the form of carbon deposited are not well understood at a basic level.
Carbon filaments are formed when a crystallite is broken away from the bulk catalyst metal, depositing a "tail" of graphitic carbon behind it (Baker et al., In Chemistry and Physics of Carbon (Edited by P. L. Walker and P. A. Thrower) Vol. 14, pg. 83-165, Marcel Dekker, New York (1978), Alstrup, J. Catal. 109, 241 (1988), and Yang et al., J. Catal. 115, 52 (1989)). Filaments are illustrated in FIGS. 1 and 2. Catalyst particle shape and graphite orientation in the filament can vary considerably (Tracz, et al., Applied Catalysis 66, 133 (1990)). The feature common to all such filaments is that carbon cones or frustra are "stacked". The graphite basal planes are not parallel to the axis of the filament, so there must be exposed carbon lattice edges at the outer circumference of each carbon layer in a filament. These represent a large number of potentially unsatisfied valences which are believed to be capped by hydrogen atoms.
A less common carbon deposit consists of multi-layer "shells" encapsulating catalyst particles. This closed form of carbon is illustrated in FIG. 3. Shells were originally found at higher reaction temperatures, as shown by Audier and co-workers (Audier et al., Carbon 19, 99 (1981)). In their work, a transition from filament to shell carbon occurred above 500.degree. C. when 0.2% H.sub.2 was present in the CO (Guinot et al., Carbon 19, 95 (1981)). Note that Audier and Guinot imply they are working in a hydrogen free system.
Jose-Yacaman et al (Jose-Yacaman et al., Appl. Phys. Latt. 62, 657 (1993)) describe the formation of nanotube-like filaments from benzene at a relatively high temperature, where the inherent chemical nature of benzene and its decomposition products controls the hydrogen activity, or availability, keeping it at a fixed and relatively low level. Thus their shells and nanotubes have a certain reasonably low number of hydrogen capped plane edges per unit carbon deposited. While this number is low, it is, however, not zero, and cannot be varied substantially as it is controlled by an inherent property of the starting material, benzene.
The control of available hydrogen has not been understood by researchers in this field as a critical parameter in the production of closed carbon structures. As an example, Tenant, U.S. Pat. No. 5,165,909, discloses the supposed production of nanotubes. However, a close examination of the conditions used by Tenant indicates that the structures being produced are actually filaments. In fact, Tenant indicates that a wide range of process parameters can be used to prepare `nanotubes`, even though it is apparent that the products produced by Tenant are not nanotubes as conventionally understood and that the patent makes indiscriminate use of the terms "filament", "fibril", and "tube".
In investigations with no hydrogen present in the reaction gas, shells and other non-filamentary carbons were formed at lower temperatures (Nolan et al., Carbon 32, 479 (1994)). In addition, as noted above, descriptions of shells and tubes formed by the high temperature catalytic decomposition of benzene appear in the literature. (Jose-Yacaman et al., Appl. Phys. Lett. 62, 657 (1993)). However, from the micrographs presented, it is apparent that these `tubes` are poorly formed intermediates between tubes and filaments, and would understandably be so due to the presence of hydrogen available from the benzene precursor at some level which cannot be reduced. The mechanism of catalytic shell carbon growth has not been elucidated. However, it is noted that Jablonski et al (Jablonski et al., Carbon 30(1), 87 (1992)) have shown that previous reports of structures prepared in the total absence of hydrogen, were in actuality formed in a reaction gas having 0.2% or more of hydrogen as an impurity, as reported by the previous workers themselves. In fact, they indicate that the reaction of 2 mol of CO to give C+CO.sub.2 has a rate of essentially zero in the absence of hydrogen.
A novel form of carbon, depicted in FIG. 4, appears as long hollow tubes, with an outer diameter commonly about 15 nm. These carbon structures are distinguished from filaments by the direction of the graphite basal planes: parallel to the axis of the tube. With hemispherical caps at their ends, these are closed forms of carbon and could be considered "higher fullerenes." In recent literature by fullerene researchers (Pang et al., J Phys. Chem. 97, 6941 (1993), Seraphin et al., Carbon 31, 685 (1993), and Ebbesen et al., Nature 367, 519 (1994)) similar but smaller (thinner) carbon formations are observed, and are often referred to as nanotubes. Nanotubes are normally produced by arc-discharge evaporation techniques designed for fullerene production (Iijima, Nature 354, 56 (1991)), but have also been produced catalytically from benzene (Jose-Yacaman et al.) or CO (Nolan et al., In Engineering, Construction, and Operations in Space IV (Edited by R. G. Galloway and S. Lokaj) Vol. 2, pg. 1199, American Society of Civil Engineers, New York (1994)). Nanotubes have the distinction of being the most resistant of the known forms of carbon to oxidation, while filaments, with their open plane edges, are far less resistant to oxidation. (Pang et al). The arc-discharge technique has been used to produce not only nanotubes, but also shells and nanoencapsulates.
A variety of conditions can affect the carbon deposit's morphology. Reaction temperature is known to be an important factor (Audier et al). Catalyst particle size and shape (Tracz et al) and the chemical nature of the catalyst (e.g. alloy) (Kim et al., J. Catal. 134, 253 (1992)) also play a role. The overall chemical reaction that the carbon containing gas mixture undergoes to produce solid carbon does not generally determine morphology. For instance: both CO disproportionation with hydrogen present and steam reforming of butane (Tracz et al) can produce similar appearing filamentous deposits. The carbon activity in the gas phase determines whether a carbide can form from a particular catalyst metal. Carbide formation in the catalyst metal is believed to have an influence on fragmentation and the creation of filaments (de Bokx et al., J Catal 96, 454 (1985)). Duration of reaction is also important, as one form of carbon can be produced at first, with another form appearing after longer reaction times (Jablonski et al). Many other effects, such as metal-support interactions (Baker et al., Carbon 21(5), 463 (1983)), can also affect carbon deposit morphology.
Unfortunately, up until now there has been no definitive method for obtaining nanotubes under controlled conditions which allow for the formation of uniform nanotubes of highly oriented graphite structure.